The present invention relates to an optical PLL (Phase-Locked Loop) circuit adopted for optical signal process in an ultra-high speed optical repeater, an ultra-high speed optical terminal, or the like, the optical PLL circuit producing a clock signal (a chain of clock pluses) which is synchronized in phase with the light-intensity modulation signal contained in a received optical signal light-intensity-modulated. The present invention also relates to a method of controlling an optical PLL circuit.
Recently, there have been strong demands for further improving high speed, large capacity communications means. The limits in operational speed of the electric circuits have made it difficult to realize higher speed transmission in the current system. Many institutes for research are now studying the optical signal processing technique which can directly process optical signals, without any change, to be adopted for the next-generation ultra-high-speed, large capacity optical communication means. Optical PLL circuits are conveniently used as optical timing extraction means important in the optical processing.
FIG. 8 illustrates the basic configuration of a conventional optical PLL circuit disclosed in JP-A 212036/1989.
In this prior art, the relation between the repetitive frequency of a received optical signal and a clock signal timing-extracted corresponds to the case of N=1 (where N is a natural number) in the present invention.
A mixer 308 receives the oscillation signal of a frequency f from a voltage-controlled oscillator 306 and the output signal of a frequency .DELTA.f from a low frequency oscillator 307 and then produces the frequency component (f+.DELTA.f) of an optical signal. An optical amplifier 302 modulates the input signal from the signal input terminal 301 under control of the frequency component. When the optical amplifier 302 receives a received optical signal of a repetitive frequency f0 and then creates an optical signal of a difference frequency component ((f+.DELTA.f)-f0). A light receiving element 303 converts the difference frequency component into an electric signal. The narrow band-pass filter 304 extracts only the low frequency component (f+.DELTA.f-f0) from the converted electric signal. In response to the output from the narrow band-pass filter 304, the frequency discriminator produces a control voltage to the voltage-controlled oscillator 306 so as to equalize the difference frequency (f+.DELTA.f-f0) to the frequency .DELTA.f.
In such a manner, when the voltage-controlled oscillator 306 is controlled, the frequency f equals the frequency f0. Hence, the voltage-controlled oscillator 306 produces an output signal which has the same repetitive frequency as that of a received optical light signal and is in synchronism with the phase thereof. The optical pulse generator 309 receives the output signal from the voltage-controlled oscillator 306 and then produces a chain of optical clock pulses which has a repetitive frequency f0 and which is in synchronism with the phase thereof.
The conventional optical PLL circuit requires a frequency mixer that creates the sum frequency (f+.DELTA.f) of the output frequency f of the voltage-controlled oscillator and the output frequency .DELTA.f of the low frequency oscillator.
However, the mixer with a normal intensity produces the frequency component (f-.DELTA.f) together with the frequency component (f+.DELTA.f), and both the frequency components are spaced at narrow frequency intervals twice the low frequency .DELTA.f. Hence, it is very difficult to realize the mixer that outputs only the frequency components (f+.DELTA.f). Such an ideal mixer is complicated in the circuit configuration. Moreover, the frequency mixing characteristics makes is difficult to realize an optical PLL circuit with high precision characteristics.